The All-Russian Nature Conservation Society will ask prosecutors to check whether opposition leader Alexei Navalny is guilty of poaching.
The ecologists' suspicions were raised by a picture published on the Internet that shows Navalny posing beside a dead moose.
"If you look at the photograph, it's clear that the hunting took place in summer," but the hunting season starts in mid-October, said Lyubov Duiko, deputy head of the society.
"Judging by the photo, illegal hunting was taking place, otherwise known as poaching," Duiko said, Rapsi news agency reported.
Navalny — clearly unfazed by the prospect of legal action being taken against him — took to his Twitter account to laugh off the reports, simply writing, "HAHAHAHAHAHA … ."
In an interview with Esquire several years ago, Navalny, who came second in September's mayoral elections, said that he loves hunting, but that he considers himself a "theoretical hunter."
"In my whole life I have only killed grouse and woodcock," he said.
Illegal hunting is punishable by up to two years in prison.
A former lawyer, Navalny has received a lot of unwanted attention from the Russian legal system in recent years for cases that many observers believe are motivated by his criticism of the Kremlin.
In October he was given a five-year suspended sentence for embezzling nearly $500,000-worth of timber from a state-owned company, KirovLes, in 2009.
Later that month, investigators accused Navalny and his brother, Oleg, of defrauding a Russian branch of French cosmetics company Yves Rocher.
In November, a Moscow court seized assets worth 7 million rubles from Alexei Navalny and his brother as part of the investigation.